A transient variation (referred to as an AC noise on power supply line) in power supply voltage, attributable to a current flowing at the time of a circuit in operation, (a current flowing at the time of transition in circuit condition, called a switching current in a CMOS circuit) occurs to a semiconductor integrated circuit. If the AC noise on power supply line is large in magnitude, this will cause malfunction of a circuit in the worst case, and there is therefore the need for suppressing the AC noise on power supply line to fall within a normally operable range.
As a margin for the AC noise on power supply line has since decreased due to reduction in voltage of a semiconductor integrated circuit, as seen lately, in particular, so has increased importance attached to suppression of variation in power supply voltage. Further, there is the need for lowering the AC noise on power supply line occurring to the semiconductor integrated circuit in consideration of an increase in importance attached to EMC (Electro-Magnetic-Compatibility).
For example, in JP-A-2001-339288, an RC filter is added to a power supply line, attempting to decrease the noise. With the RC filter, however, there occurs drop in power supply, due to current flowing to the power supply line, and a resistor as a component of the filter.
If a capacitor called a decoupling capacitor is placed between power supplies (VDD and VSS) in order to suppress the AC noise on power supply line, this will provide effective means. JP-A-2004-327820 has disclosed an example of using the decoupling capacitor. In JP-A-2004-327820, there has been proposed a design idea whereby the decoupling capacitor is connected to power supply lines during a circuit operation period, and is electrically separated from the power supply lines during a non-operation period, thereby reducing a leak current in order to cope with an increase in gate leak current accompanying process miniaturization.
FIG. 1 shows an example of a decoupling capacitor. The decoupling capacitor has a configuration wherein an NMOS transistor 1 has a gate connected to VSS, having a source and a drain, connected to VDD, respectively. Further, the decoupling capacitor has a configuration wherein a PMOS transistor 2 has a gate connected to VDD, having a source and a drain, connected to VSS, respectively. Further, “a MOS transistor” referred to in description given hereinafter may be a common field effect transistor including a MIS (Metal-Insulator-Semiconductor) transistor.
Further, progress has since been made in reduction in film thickness of a gate oxide film, accompanying miniaturization in process. In IEEE Proceedings of 7th International Symposium on Quality Electronic Design 2006, a cross-coupled decoupling capacitor as shown in FIG. 2 has been proposed for the purpose of enhancement in resistance to electrostatic breakdown occurring as a result of reduction in the film thickness. With the cross-coupled decoupling capacitor, if a gate is directly connected to VDD, VSS, respectively, there occurs deterioration in electrostatic breakdown resistance, so that with respect to MOS transistors 3, 4, respectively, source-to-drain resistance is inserted between the gate and VDD, VSS, respectively, thereby implementing enhancement in the electrostatic breakdown resistance.
The AC noise on power supply line is influenced by not only the decoupling capacitor but also electrical characteristics of a package, a PCB (Printed-Circuit-Board), and so forth. For example, in IEEE Signal Propagation on Interconnects, 2004, Proceedings, pp. 45-48, it has been proposed to suppress a voltage bounce of a power supply line, due to resonance occurring to a power source, by insertion of a resistor of a suitable resistance (the resistance is called a damping resistance).